The present invention relates generally to the field of precision motion control systems, and, in particular, to the use of relaxor ferroelectric material-based piezoelectric actuators in a closed loop motion control system.
Traditional piezoelectric ceramics such as Pb(Zr1xe2x88x92XTiX)O3 (xe2x80x9cPZTxe2x80x9d) are currently the material of choice and the mainstream for high performance actuator applications. PZT ceramics are of a morphotropic phase boundary (xe2x80x9cMPBxe2x80x9d) composition. As a result of polarizability, MPB compositions demonstrate high dielectric and piezoelectric characteristics. Consequently, PZT with MPB compositions will typically generate up to 0.2% strain levels with electric fields up to 40 kV/cm, as illustrated in FIG. 1. However, the PZT compositions will typically breakdown as electric fields are increased above 40-50 kV/cm. In addition, morphotropic PZT material exhibits significant hysteresis as it expands and retracts. FIG. 2 is a graphical illustration of a typical hysteretic curve of morphotropic PZT material without the use of the closed loop control system of the present invention.
Recent developments have established that relaxor-based single crystals possess enhanced performance characteristics compared to alternative polycrystalline forms. For example, in U.S. Pat. No. 5,998,910 to Park et al. (xe2x80x9cthe ""910 patentxe2x80x9d), single crystals of Pb(Mg1/3Nb2/3)O3 (xe2x80x9cPMNxe2x80x9d), Pb(Zn1/3Nb2/3)O3 (xe2x80x9cPZNxe2x80x9d), and their solid solutions with PbTiO3 (xe2x80x9cPTxe2x80x9d) have been shown to exhibit electric field induced strains greater than 1%, longitudinal coupling coefficients (k33) greater than 90, piezoelectric coefficients (d33) greater than 2000 pc/N, and dielectric constants from 1000-5000 with low dielectric loss (less than 1%). FIG. 1 is a graphical representation comparing the relationship between the strain and electric field behavior for various crystals of PZN-PT and PMN-PT to that of various electromechanical ceramics. As illustrated, the relaxor crystals defined in the ""910 patent maintain higher strain rates, and the dielectric properties of these materials collectively demonstrate great potential for expanding existing actuator applications.
The new capability opened up by these new materials makes them ideally suited for small to medium scale engineering applications, often referred to as xe2x80x9cmeso-scalexe2x80x9d systems. Meso-scale systems incorporate mechanisms such as the components in a mechanical watch, the small suspensions for supporting magnetic read heads in computer disc drives, miniature systems for minimally-invasive medical procedures, and many others. Although traditional piezoelectric materials such PZT have long been used to provide precise motion control, their applicability to meso-scale systems has always been very limited because their limited strains make the materials much less effective when used to drive motions of more than a small fraction of a millimeter. Lever mechanisms may be used to increase the range of traditional piezo-electric materials, but the resulting actuators have limited load bearing capacity, are bulky and have a low dynamic response.
By comparison, the large strains of which the relaxor materials are capable make it possible to produce actuators that potentially allow sub-nanometer resolution over ranges on the order of millimeters or more using only a single device, rather than through the combination of multiple devices. This in turn would enable the realization of motion control systems with motions ranging from micro-scale (micrometers or even nanometers) to meso-scale (tens of micrometers to millimeters), enabling the development of entire new families of applications.
Unfortunately, hysteresis continues to be observed for relaxor ferroelectric materials. FIG. 3 is a graphical illustration of a typical hysteretic curve of morphotropic PZN material without the use of the closed loop control system of the present invention. Although the hysteretic response of the PZN material is reduced as compared to that of some traditional piezoelectric materials, it is clearly not eliminated, as evidenced in FIG. 3. As is well known, hysteresis has a number of detrimental effects, including significant imprecision. This, combined with variations in the response of the material with temperature, self-heating (particularly at high frequencies), and other problems significantly limit the otherwise beneficial effects of actuators formed from relaxor materials. Thus, a need exists for a means for minimizing the hysteretic and other undesirable effects in relaxor actuators, thereby improving their precision and accuracy.
Existing applications for relaxor actuators, which include the incorporation of relaxor materials in ink-jet printer heads as well as ultrasonic and sonar applications, fail to meet this need. None of these applications involves the measurement of displacements for motion control. Instead, in each of these applications, relaxor materials are utilized because of the very high strains that are possible and because they can deliver a relatively high power density for enhancing signal levels in sonar, ink jet pressures/performance, and ultrasonic intensities. Thus, it is sufficient to operate known relaxor actuators in open-loop mode.
It is an object of the present invention to provide a system to enhance motion and positioning of relaxor actuators by applying these type materials to closed-loop control systems.
It is a further object of the present invention to improve upon the precision and accuracy of positioning of meso-scale systems.
Still other objectives and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.
To these ends, the present invention provides an apparatus for controlling precision motion with relaxor actuators using dynamic control. The servo system utilizes a feed back signal to maintain precise motion control and measures relative displacements or rotations of component parts of the mechanism. Broadly defined, an electromechanical actuator system according to one embodiment of the present invention includes: at least one actuator device having an actuator body formed from a relaxor-based piezoelectric material and a mechanical system driven by the actuator body; a sensor for sensing a phenomenon produced by the mechanical system in response to piezoelectric movement of the actuator body; and a driver, having an input connected to the sensor, for applying a variable electric field to the actuator body.
In features of this aspect, the sensor produces an output corresponding to the phenomenon produced by the mechanical system, and the driver applies the electric field in response to the sensor output; piezoelectric movement of the actuator body causes a first surface to move relative to a second surface, the sensor measures the separation between the first and second surfaces, and the output of the sensor is a function of the separation measurement; the sensor measures the separation between a point on the first surface and a point on the second surface; the first surface defines a first plane and the second surface defines a second plane, and the sensor measures the separation between the first plane and the second plane; the piezoelectric movement of the actuator body defines a direction of movement, and the direction of movement intersects the first and second surfaces; the actuator body defines a central axis parallel to the direction of piezoelectric movement, and the central axis intersects the first and second surfaces; the magnitude of the piezoelectric movement of the actuator body is substantially equivalent to the magnitude of change caused by the piezoelectric movement in the separation between the first and second surfaces; the sensor is disposed generally adjacent to the actuator body; the mechanical system includes a first moving member in direct physical contact with the actuator body, the area of contact generally defines a first location in the mechanical system, and the movement of the first surface relative to the second surface occurs in a second location in the mechanical system; the mechanical system includes a lever, and the movement of the first surface relative to the second surface is transmitted from the actuator body via the lever; the phenomenon produced by the mechanical system is a displacement; the displacement is the displacement of at least a portion of the mechanical system; and the displacement is a linear or rotational displacement.
In other features of this aspect, the sensor is a proximity probe; the sensor is a strain gage; the sensor detects a phenomenon other than displacement; the system further includes a comparator for comparing the sensor output to a signal representing a desired state; the system further includes a controller for controlling the driver to variably apply the electric field to the actuator body; the controller controls the driver in response to the output produced by the sensor; the system further includes at least a second driver for applying a variable electric field to the actuator body; the controller also controls the second driver to variably apply an electric field to the actuator body; the system further includes a second controller for controlling the second driver to variably apply the electric field to the actuator body; the system further includes at least a second sensor for sensing a second phenomenon produced by the mechanical system in response to the piezoelectric movement of the actuator body, and the second sensor produces an output corresponding to the second phenomenon produced by the mechanical system; the driver applies the electric field in response to the second sensor output; at least one sensor is a displacement sensor and at least one sensor is not a displacement sensor; the first sensor is not a displacement sensor and the second sensor is a displacement sensor, and the second phenomenon is a displacement caused by the piezoelectric movement of the actuator body in response to the output of the first sensor; at least two sensors are displacement sensors; at least a second actuator device has an actuator body formed from a relaxor-based piezoelectric material and a mechanical system driven by the actuator body; the system further includes a first support structure and a second support structure, the actuator body is supported by the first support structure and the sensor is supported by the second support structure, and the first and second support structures are structurally independent of each other.
In other features of this aspect, the relaxor-based piezoelectric material is a relaxor ferroelectric material; the relaxor-based piezoelectric material is PMN, PZN, a solid solution of PMN and PT, or a solid solution of PZN and PT; the relaxor-based piezoelectric material is a solid solution of PZN and PT, wherein PT is 8%; the relaxor-based piezoelectric material is a solid solution of PZN and PT, wherein PT is 4.5%; the relaxor-based piezoelectric material is a solid solution of PMN and PT, wherein PT is 24%; the actuator body is rectilinear in shape; the actuator body is curvilinear in shape; and the actuator body is formed from at least two bi-morph elements.
In another aspect of the present invention, a method of controlling an electromechanical actuator system includes the steps of: applying an electric field to an actuator body formed from a relaxor-based material to generate a piezoelectric movement in the actuator body; in response to the piezoelectric movement of the actuator body, producing a phenomenon in a mechanical system; sensing the phenomenon produced by the mechanical system; and varying the applied electric field according to the outcome of the sensing step.
In features of this aspect, the method further includes the step of generating an output signal corresponding to the phenomenon produced by the mechanical system; the method further includes the step of comparing the output signal to a signal representing a desired state; the step of varying the applied electric field is carried out according to the outcome of the comparing step; the step of producing a phenomenon includes causing a first surface to move relative to a second surface, and the step of sensing includes measuring the separation between the first and second surfaces; the step of causing a first surface to move relative to a second surface includes causing the first surface to move in a first direction defining an axis, and the axis intersects the first and second surfaces; the magnitude of the piezoelectric movement of the actuator body is substantially equivalent to the magnitude of change caused by the piezoelectric movement in the separation between the first and second surfaces; the sensing step is carried out in a location generally adjacent to the actuator body; the step of producing a phenomenon in the mechanical system includes physically contacting a first moving member of the mechanical system with the actuator body, the area of physical contact between the first moving member and the actuator body generally defines a first location in the mechanical system, and the movement of the first surface relative to the second surface occurs in a second location in the mechanical system; the mechanical system includes a lever, and the step of causing movement of the first surface relative to the second surface includes transmitting movement from the actuator body via the lever; the step of producing a phenomenon includes producing a displacement; the step of producing a displacement includes producing a displacement of at least a portion of the mechanical system; the step of producing a displacement includes producing a linear displacement; the step of producing a displacement includes producing a rotational displacement; the step of sensing includes sensing the proximity of a surface to the tip of a proximity probe; the step of sensing includes measuring strain; and the step of sensing includes detecting a phenomenon other than displacement.
In yet another aspect of the present invention, an electromechanical actuator system includes: an actuator device having an actuator body formed from a relaxor-based piezoelectric material and a mechanical system driven by the actuator body; a driver for variably applying an electric field to the actuator body; and a feedback loop for providing input to the driver in response to at least one phenomenon produced by the mechanical system.
In features of this aspect, the phenomenon produced by the mechanical system is in response to piezoelectric movement of the actuator body, and the driver applies the electric field in response to the input from the feedback loop; piezoelectric movement of the actuator body causes a first surface to move relative to a second surface, and the feedback loop provides input to the driver regarding the separation between the first and second surfaces; the input provided regarding the separation between the first and second surfaces is based on a measurement of the separation between the first and second surfaces; the piezoelectric movement of the actuator body defines a direction of movement, and the direction of movement intersects the first and second surfaces; the mechanical system includes a first moving member in direct physical contact with the actuator body, the area of contact generally defines a first location in the mechanical system, and the movement of the first surface relative to the second surface occurs in a second location in the mechanical system; the mechanical system includes a lever, and the movement of the first surface relative to the second surface is transmitted from the actuator body via the lever; the phenomenon produced by the mechanical system is a displacement of at least a portion of the mechanical system; the feedback loop provides input to the driver regarding a phenomenon other than displacement; the system further includes a comparator for comparing input from the feedback loop to a signal representing a desired state; the system further includes a controller for controlling the driver to variably apply the electric field to the actuator body in response to the input from the feedback loop; the system further includes at least a second driver for applying a variable electric field to the actuator body; the feedback loop further provides input to the driver in response to at least a second phenomenon produced by the mechanical system, and the second phenomenon is produced in response to piezoelectric movement of the actuator body; and the system further includes at least a second actuator device having an actuator body formed from a relaxor-based piezoelectric material and a mechanical system driven by the actuator body.